heftiness vs sinew

heftiness and sinew both are nouns.

Word	Noun	Adjective	Verb	Adverb
heftiness	Yes	No	No	No
sinew	Yes	No	No	No

As nouns, heftiness and sinew are synonyms defined as:

Other synonyms of heftiness include brawn, brawniness, muscle, muscularity.

heftiness (noun)	sinew (noun)
possessing muscular strength	possessing muscular strength
the property of being large in mass	a cord or band of inelastic tissue connecting a muscle with its bony attachment